Jadeite–chloritoid–glaucophane–lawsonite blueschists in north-west Turkey: unusually high P/T ratios in continental crust

A. I . OKAY_IIstanbul Teknik Universitesi, Avrasya Yerbilimleri Enstitusu, Ayazaga 80626 _IIstanbul, Turkey (okay@itu.edu.tr)

ABSTRACT Sodic metapelites with jadeite, chloritoid, glaucophane and lawsonite form a coherent regionalmetamorphic sequence, several tens of square kilometres in size, and over a kilometre thick, in theOrhaneli region of northwest Turkey. The low-variance mineral assemblage in the sodic metapelites isquartz + phengite + jadeite + glaucophane + chloritoid + lawsonite. The associated metabasitesare characterized by sodic amphibole + lawsonite ± garnet paragenesis. The stable coexistence ofjadeite + chloritoid + glaucophane + lawsonite, not reported before, indicates metamorphic pres-sures of 24 ± 3 kbar and temperatures of 430 ± 30 �C for the peak blueschist facies conditions. TheseP–T conditions correspond to a geotherm of 5 �C km)1, one of the lowest recorded in continentalcrustal rocks. The low geotherm, and the known rate of convergence during the Cretaceous subductionsuggest low shear stresses at the top of the downgoing continental slab.

Key words: jadeite, chloritoid, blueschist, geotherm, Turkey, subduction.

INTRODUCTION

Petrological data provide the best record of the thermalstructure of the subducted former continental oroceanic crust, represented as blueschist and eclogiteterranes in orogenic belts. Knowledge of the shear andradioactive heating, rate of subduction and thermalstructure of the former subduction zones depends to alarge extent on the precise and accurate characterisationof the peak P–T conditions and P–T–t paths of thedown-going slabs (e.g. Peacock, 1992). A commonproblem in this context is the estimation of metamor-phic pressure in the blueschist belts, particularly in thelawsonite–blueschist facies, which covers a large P–Tfield above c. 8 kbar and below c. 500 �C (Evans, 1990).Metabasites in lawsonite–blueschist facies are domin-ated by the low variance sodic amphibole + lawsonite± sodic pyroxene assemblages, which usually allowonly a minimum estimate of the metamorphic pressuresbased on the jadeite content of the sodic pyroxene.Metapelitic lithologies, rare in the blueschist facies, havebeen shown to have great value in the precise estimationof the peak P–T conditions in the eclogite facies (e.g.Chopin, 1981a; Schreyer, 1985). Here, I describe sodicmetapelites from northwest Turkey, which containcoexisting jadeite and chloritoid along with glauco-phane and lawsonite. The stable coexistence of jadeiteand chloritoid indicates geotherms as low as 5 �C ⁄kmfor the Cretaceous blueschist metamorphism withimplications for the subducted continental crust.Blueschists in northwest Turkey form an east–west

trending metamorphic belt, c. 250 km long andc. 50 km wide, called the Tavsanlı Zone, immediatelysouth of the main Neo-Tethyan suture (Fig. 1). The

coherent blueschist sequence in the Tavsanlı Zoneconsists of metapelitic schists at the base, marbles inthe middle and a series of metabasite, metachert andphyllite at the top (Okay, 1984). The blueschists rep-resent a subducted and subsequently exhumed passivecontinental margin sequence. Stratigraphic compar-ison with unmetamorphosed sequences farther south inthe Taurides suggests a Infra-Cambrian–Palaeozoicdepositional age for the basal metaclastic rocks, and aMesozoic age for the marbles and the overlyingmetabasite-metachert sequence. Phengite Rb–Sr andAr–Ar data from the blueschists indicate a Late Cre-taceous (80 ± 5 Ma) age for the HP ⁄LT metamor-phism (Sherlock et al., 1999). The coherent blueschistsequence is tectonically overlain by a Cretaceousaccretionary complex of basalt, radiolarian chert andpelagic shale. The accretionary complex exhibits gen-erally a low-grade incipient blueschist metamorphism.Large tectonic slabs of ophiolite, predominantlyperidotite, lie over the coherent blueschists or theaccretionary complex (Okay, 1984).

GEOLOGICAL SETTING

Chloritoid–jadeite schists occur in the western part ofthe Tavsanlı Zone around the town of Orhaneli.Blueschists in the Orhaneli region are predominantlymetapelitic schists and marbles. The schists consistof quartz-rich greyschists at the base, and white,phengite-rich schists at the top below the marbles(Lisenbee, 1971). The greyschists form grey, finelybanded, hard, medium-grained rocks representingsodic metapelites (Okay & Kelley, 1994). The mini-mum thickness of the schist sequence is c. 800 m. The

J. metamorphic Geol., 2002, 20, 757–768

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schists are overlain by white marbles of over 400 m inthickness. Marble and calcschist bands, several metresthick, also occur within the schists. The schists show astrong ductile deformation with the development ofpenetrative foliation and mineral lineation. Isoclinalfolds are present from the scale of thin section to200–300 m with generally east–west trending fold axis(Lisenbee, 1971). Thin marble layers in the schists arestretched out in the fold limbs and are seldom con-tinuous for more than a few hundred metres. Themineral stretching lineation defined by sodic amphi-bole and chloritoid in the schists, and by calcite in themarbles trends parallel to the axis of the isoclinalfolds. Rare metabasite layers interfolded with thecalcschists occur north of the village of Deliballar(Fig. 2). A late crenulation cleavage is well developedin the micaceous lithologies.The coherent blueschist sequence is tectonically

overlain by the Burhan ophiolite, which is made upof north–south trending dunite, harzburgite andgabbro bands, up to several kilometres in thickness(Lisenbee, 1971). The peridotite is cut by an east–

west trending swarm of dolerite dykes. Narrowtectonic slivers of garnet-amphibolite, similar to themetamorphic soles described from the base of ophi-olites worldwide (e.g. Spray, 1984), occur at the baseof the Burhan ophiolite (Fig. 2). Locally, the sub-ophiolite metamorphic rocks show a weak blueschistfacies overprint marked by development of smalllawsonite aggregates in plagioclase, and by thin rimsof sodic amphibole around hornblende crystals(Okay et al., 1998; Onen & Hall, 2000). Hornblendefrom the garnet–amphibolite north of Deliballar hasbeen dated by Ar–Ar method as 101 ± 4 Ma(Harris et al., 1994).The blueschists in the Orhaneli region are cut by twoEocene granodioritic plutons with well-marked contactmetamorphic aureoles (Fig. 2). The Orhaneli andTopuk granodiorites are undeformed post-tectonicintrusions dated as 53 and 48 Ma, respectively(Harris et al., 1994). Blueschist minerals are com-pletely destroyed within a one-kilometre-wide contactmetamorphic aureole with the growth of new minerals,including andalusite, cordierite, alkali feldspar and

Fig. 1. Tectonic map of northwest Turkey showing the regional distribution of the blueschists.

7 58 A . I . O K A Y

biotite. The contact metamorphic mineral assemblagesand the Al-content of amphibole in the Orhaneligranodiorite indicate that the pluton was emplaced inthe blueschists at a depth of c. 10 km during the Early

Eocene (Harris et al., 1994). The blueschists, theophiolite and the granodiorites are unconformablyoverlain by continental Miocene sedimentary andvolcanic rocks.

Fig. 2. Geological map and cross-section of the Orhaneli region (modified from Lisenbee, 1971) with the sample locations.The sample numbers are keyed to Table 1. For location see Fig. 1.

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PETROLOGY AND MINERAL CHEMISTRY

Petrography

One hundred and eighteen schist samples were petro-graphically studied from the Orhaneli region. Thecommon minerals in the greyschists, in order ofabundance, are quartz, white mica, jadeite, chloritoid,sodic amphibole, lawsonite and chlorite. Minorminerals include tourmaline, apatite, rutile, graphite,retrograde albite and magnetite.Quartz and white mica constitute over half of the

mode in the greyschists (Table 1). Quartz grainscommonly show undulose extinction and recrystal-lization to smaller grain aggregates. White mica,predominantly phengite, forms aggregates of slendercrystals defining the foliation. Only one out of 10analysed samples contained paragonite closely asso-ciated with phengite, similar to the textures describedby Shau et al. (1991) and Okay & Kelley (1994).Jadeite commonly occurs as equant porphyroblastsup to 1 mm in length. In many sections it appearsdark due to small opaque inclusions (Fig. 3). Insome sections jadeite porphyroblasts are rimmed andpartially replaced by sericitic white mica. Bluishgreen chloritoid forms elongate prismatic grains,c. 0.5 mm long, aligned parallel to the foliation.Sodic amphibole occurs as idioblastic crystals, c.0.5 mm long. In several samples it shows zoning withpale blue cores and colourless rims. Lawsonite showsits typical prismatic habit. In many thin-sections thefour minerals, jadeite, chloritoid, sodic amphiboleand lawsonite, occur in close proximity and areinferred to be in textural equilibrium (Fig. 3). In theleast retrograded samples, such as 5080, chlorite oroxy-chlorite are absent or very scarce. In othersamples pale-green chlorite or reddish-brown oxy-chlorite can be seen replacing sodic amphibole orchloritoid (Fig. 3b), or grow along late crenulation

cleavages. Therefore, they are inferred to be lateminerals not in equilibrium with the high-pressurephases.

Table 1. Estimated modal amounts of the analysed blueschist samples.

Greyschist (metapelite) Metadiorite Metabasite

4892A 4892B 4893B 5038B 5040 5079 5080 5086 5366 104B 4291B

Quartz 19 47 49 38 49 41 43 33 34 1 12

White mica 58ph,pa 15ph 9ph 23ph 13ph 18ph 11ph 21ph – 12ta –

Jadeite – 14 23 19 18 15 21 – 41 trcl –

Chloritoid 8 12 3 5 6 5 11 4 3 – –

Na-amphibole 2 1 6 – 5 4 14 17 14 38 41

Lawsonite 3 5 6 – 2 3 – 15 3 42 45

Garnet – – – – – – – – – 4 –

Rutile – tr tr 1 tr – tr – tr tr –

Titanite – – – – – – – – – – 2

Opaque – tr tr tr trpy,ilm – tr trmag 2 – tr

Tourmaline tr tr tr – tr tr tr tr – tr –

Apatite tr tr – tr tr – – – – tr –

Graphite 1 tr tr tr tr 2 tr 2 – – –

Chlorite 5 2 1 8 7 6 – 2 – 3 –

Oxy-chlorite 4 4 3 – – 3 tr 6 – – –

Albite tr tr tr 4 – 3 – – 3 – –

Ankerite – – – 2 – – – – – – –

ph, phengite; pa, paragonite; ta, talc; py, pyrite; ilm, ilmenite; mag, magnetite; cl, chloromelanite. tr < 0.5.

Fig. 3. Photomicrographs of jadeite-chloritoid paragenesis inthe Orhaneli region, plane polarized light. (a) Greyschist (5080)with jadeite (jd), chloritoid (ctd), glaucophane (gl), phengite (ph)and quartz (qz). (b) Greyschist (4893B) with (jd) (ctd) (gl) (lw)(ph) (qz) and tourmaline (tour). Chloritoid is partially replacedby oxy-chlorite (o-chl).

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A very hard massive rock with a ghost magmatictexture forms a-few-hundred-metre long and c. 20 mwide elongate body in the greyschists southeast of thetown of Orhaneli. It consists essentially of quartz andjadeite with minor amounts of glaucophane, chloritoidand lawsonite. The rock, probably a former dioriticdyke, is comparable to the jadeite-rocks described25 km farther west from the Kocasu region (Okay &Kelley, 1994).Metabasites around Deliballar consist mainly of

lawsonite and sodic amphibole with minor sodic pyr-oxene, chlorite, white mica and titanite, and are petro-graphically similar to the blueschist metabasitesdescribed from north-east of Tavsanlı, 80 km farthereast (Okay, 1980). Rarely, small pink garnet occurs inthe blueschist metabasites coexisting with lawsonite andsodic amphibole.

Mineral chemistry

Eight greyschists, one metadiorite and two metabasitesamples were studied with an electron microprobe.Samples 4892 A, B and 4893B were analysed in theUniversite Paris 6 and the rest in the Open Universityusing an SX-50 Cameca electron microprobe. Theoperating conditions for the microprobe were 15 kVaccelerating voltage, 15 nA beam current and 10 lmbeam size. Estimated modes of these samples aregiven in Table 1 and representative mineral compo-sitions in Tables 2 and 3. The Fe3+ in sodic

amphibole was estimated on the basis of structuralformulae of 23 oxygen and 15 cations. Structuralformulae of jadeite were calculated on the basis offour cations. The jadeite component is taken to beequal to Al6; the aegerine component equals Na ⁄(Na + Ca)-Al6. The rest is assigned to the augitecomponent.Sodic pyroxene from the greyschists and the met-

adiorite is essentially endmember jadeite with thejadeite component ranging from 82 to 98 mol percent(Fig. 4, Table 2). Sodic amphibole from the meta-clastic rocks is ferroglaucophane and glaucophanelargely free of Fe3+, as shown by the cation totals of<15 for a 23 oxygen formula. The zoning in sodicamphibole involves an increase in theMg ⁄ (Fe2+ + Mg) ratio towards the rim (Fig. 5,Table 2). Sodic amphibole from the metabasites isglaucophane and crossite. Analysed lawsonite is veryclose to the ideal lawsonite composition. Chloritoidfrom the greyschists is iron-rich with a restrictedFe ⁄ (Fe + Mg) ratio of 0.83–0.92, whereas in themetadiorite it is slightly richer in magnesium. The Siper-formula-unit (p.f.u) in the analysed phengiteranges from 3.45 to 3.55 (Fig. 6). There is little vari-ation in the K (0.86–0.93 p.f.u) and Na (0.013–0.037)contents of the analysed phengite. Only one sample(4892 A) contains stably coexisting paragonite andphengite. Paragonite is close to the endmember com-position (Table 2). Oxy-chlorite contains higher SiO2and K2O and lower Al2O3 contents than chlorite

Table 2. Representative mineral compositions from the greyschists.

4892A 4893B 5080

ctd law gl ph pa chl ctd jadeite gl ph o-chl ctd jadeite

gl

core

gl

rim ph

SiO2 24.31 38.34 57.18 52.17 47.32 25.47 24.29 58.97 56.49 52.68 26.72 24.51 59.13 56.81 58.69 52.08

TiO2 0.00 0.02 0.10 0.10 0.08 0.07 0.00 0.03 0.14 0.15 0.06 0.04 0.04 0.22 0.01 0.09

Al2O3 40.94 31.86 11.97 25.23 38.94 20.53 40.67 21.70 11.50 24.51 20.56 40.32 22.82 11.00 11.54 24.27

Cr2O3 0.02 0.03 0.04 0.05 0.00 0.00 0.01 0.00 0.02 0.09 0.08 n.d. n.d. n.d. n.d. n.d.

FeO 25.15 0.22 14.56 2.77 0.51 31.15 25.11 4.61 16.55 3.53 31.64 24.40 2.66 16.43 12.16 2.65

MgO 1.89 0.02 6.11 3.18 0.24 9.35 1.40 0.22 5.14 3.07 7.06 2.53 0.15 6.18 8.42 3.64

MnO 0.32 0.00 0.00 0.02 0.00 0.00 0.36 0.01 0.00 0.00 0.30 0.59 0.00 0.10 0.03 0.03

CaO 0.04 16.98 0.03 0.10 0.09 0.12 0.02 0.40 0.10 0.00 0.15 0.00 0.24 0.07 0.02 0.00

Na2O 0.00 0.00 7.49 0.11 7.74 0.03 0.03 14.62 6.86 0.22 0.09 0.00 14.81 6.85 7.12 0.17

K2O 0.04 0.01 0.05 10.35 0.74 0.10 0.00 0.00 0.07 10.05 0.74 0.00 0.04 0.01 0.00 10.32

Total 92.71 87.48 97.53 94.08 95.67 86.82 91.89 100.56 96.87 94.30 87.40 92.39 99.89 97.67 97.99 93.25

Oxygen 12 8 23 11 11 14 12 4 cat. 23 11 14 12 4 cat. 23 23 11

Si 2.01 2.02 7.99 3.52 3.02 2.77 2.02 2.02 8.03 3.55 2.90 2.03 2.01 8.01 8.06 3.55

Al4 0.00 0.00 0.01 0.48 0.98 1.23 0.00 0.00 0.00 0.45 1.09 0.00 0.00 0.00 0.00 0.45

Al6 3.99 1.98 1.96 1.53 1.95 1.44 4.00 0.87 1.93 1.50 1.58 3.93 0.92 1.83 1.87 1.50

Ti 0.00 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.02 0.01 0.00 0.00 0.00 0.02 0.00 0.01

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01

Fe3+ 0.01 0.01 0.06 0.00 0.12 0.00 0.07 0.07 0.00 0.00

Fe2+ 1.73 0.00 1.64 0.16 0.03 2.88 1.75 0.01 1.97 0.20 2.91 1.62 0.01 1.94 1.40 0.15

Mg 0.23 0.00 1.28 0.32 0.02 1.54 0.17 0.01 1.09 0.31 1.16 0.31 0.01 1.30 1.72 0.37

Mn 0.02 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.03 0.04 0.00 0.01 0.00 0.00

Ca 0.01 0.96 0.00 0.01 0.01 0.01 0.00 0.01 0.02 0.00 0.02 0.00 0.01 0.01 0.00 0.00

Na 0.00 0.00 2.03 0.01 0.96 0.01 0.00 0.96 1.89 0.03 0.02 0.00 0.97 1.87 1.90 0.02

K 0.00 0.00 0.01 0.89 0.06 0.01 0.00 0.00 0.01 0.86 0.10 0.00 0.00 0.00 0.00 0.90

Total 8.00 4.97 15.00 6.92 7.03 9.90 7.97 4.00 14.96 6.91 9.82 8.00 4.00 14.99 14.95 6.95

Activities at 24 kbar and 430 �C:fct 0.88 gl 0.135 mu 0.35 pa 0.91 fct 0.90 jd 0.85 gl 0.089 mu 0.31 fct 0.83 jd 0.90 gl 0.09 gl 0.22 mu 0.32

mct 0.14 fgl 0.25 cel 0.17 mct 0.11 fgl 0.32 cel 0.16 mct 0.18 fgl 0.21 fgl 0.142 cel 0.20

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(Table 2) and is probable similar to the chlorite ⁄biotitemixed layer silicate described as an alteration productof chlorite by Maresch et al. (1985). One of the ana-lysed metabasite samples contains talc, which coexistswith garnet, sodic amphibole and lawsonite. Garnet inthis sample forms small subidioblastic grains,<0.4 mm across, and is essentially an almandine-grossular-spessartine solid solution with minor pyrope

Table 3. Mineral compositions from the metadiorite and metabasites.

5366 UL104B 4291B

ctd jadeite gl law gl law

grt

core

grt

rim talc gl law

SiO2 24.64 58.56 58.38 37.64 56.96 38.16 36.58 36.84 59.85 56.40 38.31

TiO2 0.00 0.00 0.14 0.00 0.00 0.03 0.00 0.00 0.00 0.18 0.13

Al2O3 42.26 22.28 12.20 32.47 5.02 30.95 20.17 20.43 0.10 8.66 31.42

Cr2O3 0.00 0.01 0.00 0.00 0.04 0.43 0.00 0.08 0.00 0.00 0.06

FeO 20.84 1.98 8.54 0.00 17.60 1.76 21.31 24.58 9.28 14.46 1.10

MgO 3.94 1.23 10.23 0.00 9.94 0.00 0.84 0.85 24.24 8.64 0.00

MnO 0.56 0.08 0.01 0.00 0.04 0.02 12.81 9.84 0.00 0.32 0.03

CaO 0.00 1.64 0.25 16.96 1.51 17.06 8.29 7.37 0.02 0.46 17.00

Na2O 0.02 13.50 7.45 0.00 6.22 0.00 0.00 0.00 0.00 7.11 0.04

K2O 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.07

Total 92.26 99.28 97.21 87.07 97.33 88.41 100.00 99.99 93.50 96.23 88.16

Oxygen 12 4 cat. 23 8 23 8 12 12 11 23 8

Si 2.00 2.01 7.96 1.99 8.05 2.02 2.95 2.98 4.02 7.96 2.02

Al4 0.00 0.00 0.04 0.01 0.00 0.00 0.05 0.02 0.00 0.04 0.00

Al6 4.04 0.90 1.92 2.02 0.84 1.93 1.87 1.92 0.01 1.40 1.95

Ti 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.01

Cr 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00

Fe3+ 0.00 0.04 0.05 0.00 0.78 0.05 0.13 0.08 0.53 0.04

Fe2+ 1.42 0.02 0.93 0.02 1.30 0.03 1.31 1.58 0.52 1.18 0.01

Mg 0.48 0.06 2.08 0.00 2.10 0.00 0.10 0.10 2.43 1.82 0.00

Mn 0.04 0.00 0.00 0.00 0.01 0.00 0.88 0.67 0.00 0.04 0.00

Ca 0.00 0.06 0.04 0.96 0.22 0.97 0.72 0.64 0.00 0.07 0.96

Na 0.00 0.91 1.97 0.00 1.70 0.00 0.00 0.00 0.00 1.94 0.00

K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 7.98 4.00 15.00 5.00 15.00 5.02 8.01 7.99 6.98 15.00 4.99

Activities at 24 kbar and 430 �C:fct 0.75 jd 0.84 gl 0.33 gl 0.054 alm 0.073 alm 0.14 ta 0.55

mct 0.28 fgl 0.06 fgl 0.014 py 0.00016 py 0.00015 fta 0.0052

gr 0.011 gr 0.0085

Fig. 4. Jadeite composition from the Orhaneli area plotted in thejadeite corner of the aegerine (NaFeSi2O6)-jadeite (NaAlSi2O6)-augite (Ca(MgFe)Si2O6) ternary diagram.

Fe 2+

(Fe

2++

Mg)

Fe 2+

(Fe

2++

Mg)

Fig. 5. Sodic amphibole compositions from the Orhaneli regionplotted on part of the Miyashiro diagram. Arrows indicate coreto rim compositions. Fg, ferro-glaucophane; Gl, glaucophane;Mr, magnesio-riebeckite; Ri, riebeckite. Other symbols as inFig. 4.

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endmember (3–4 mol percentage). The garnet shows agrowth zoning with an increase in Fe and Fe ⁄Mg ratioand a decrease in Mn and Ca towards the rim withlittle variation in Mg (Table 3).The regular Fe2 ± Mg partitioning among sodic

amphibole, phengite and chloritoid in the greyschists,despite the restricted Fe ⁄ (Fe + Mg) range, suggeststhat the analysed compositions represent equilibriumor near-equilibrium assemblages (Fig. 7). The averageFe2+ ⁄Mg partition coefficient KD for chloritoid ⁄ sodicamphibole, chloritoid ⁄ phengite and sodic amphi-bole ⁄ phengite from nine samples are 5.6, 13.1 and 2.4,respectively. The Fe2 ⁄Mg distribution coefficient KDbetween sodic amphibole and chloritoid in the Orha-neli region is the lowest reported in the literature (cf.Theye & Seidel, 1991). This might reflect the high P ⁄Tratio in the region, as incorporation of Mg into chlo-ritoid will be favoured by increasing pressure (Chopin,1981a).

PHASE RELATIONS AND P–T CONDITIONS

In the Orhaneli greyschists the low-variance blueschistfacies mineral assemblage is sodic amphibole + chlo-ritoid + jadeite + lawsonite + quartz + phengite +rutile ± paragonite. Phengite, lawsonite and rutile arethe only major K-, Ca- and Ti-bearing phases, and sodicamphibole and chloritoid have very low Fe3+ contents,so that the sodic amphibole, chloritoid, jadeite andparagonite lie, to a first approximation, in the Na2O–FeO–MgO–Al2O3–SiO2–H2O system (NFMASH).High-pressure mineral equilibria in the NFMASH sys-tem have been investigated by Guiraud et al. (1990) andEl-Shazly & Liou (1991). Both studies assign pressuresof above 18 kbar, and temperatures of <580 �C to thejadeite + chloritoid assemblage, regardless of the

composition of the chloritoid. In the 400–580 �C tem-perature range, the jadeite + chloritoid assemblage islimited at low pressures by the near isobaric reaction(Fig. 9):

jadeite þ chloritoid þ quartz þ H2O¼ paragonite þ sodic amphibole ð1Þ

Paragonite and sodic amphibole is a common assem-blage in the high-pressure rocks in both pelitic andmafic rock compositions (cf. Mollini & Poli, 1997). Ithas been described among others from Cyclades,Greece (e.g. Schliestedt, 1986; Okrusch & Brocker,1990), Alps (e.g. Holland, 1979; Oberhansli, 1986;Heinrich, 1986), Turkey (Okay, 1989; Okay & Kelley,1994), Oman (e.g. El-Shazly & Liou, 1991), Himalaya(Guillot et al., 1997), Alaska (Patrick & Evans, 1989)and Venezuela (Sisson et al., 1997). In contrast, jade-ite + chloritoid, the high pressure equivalent of par-agonite + sodic amphibole, has only been describedfrom inclusions in garnet in the Cyclades and in theHimalaya (Schliestedt & Okrusch, 1988; Guillot et al.,1997).The presence of lawsonite + jadeite + rutile, and

absence of coesite designate a broad P–T field for the

Fig. 6. Phengite compositions from the Orhaneli region shownin terms of Si and (Fe + Mg + Mn) per formula unit (p.f.u).The ideal celadonite-muscovite substitution line is also shown.

Fig. 7. Distribution of Fe2+ ⁄ (Fe2++Mg) among chloritoid,sodic amphibole and phengite. Only rim compositions of sodicamphibole are shown.

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blueschists in the Orhaneli region on the basis of thereactions:

jadeite þ lawsonite þ rutile¼ paragonite þ titanite þ H2O ð2Þ

quartz ¼ coesite ð3ÞLow-variance mineral assemblages in the greyschists inthe Orhaneli region have a potential of accuratelyconstraining the P–T conditions of the blueschistmetamorphism. Mineral equilibria in the analysedblueschists in the Orhaneli region were calculated usingthe THERMOCALC program (v. 2.75) of Powell &Holland (1988) with the thermodynamic data set ofHolland & Powell (1998). The activities used in equi-libria leading to the P–T estimation are shown inTables 2 and 3. They were obtained using the AXprogram of T.J.B. Holland (www.esc.cam.ac.uk/astaff/holland). The program uses ideal site mixing fortalc and sodic amphibole, two-site nonideal mixingfor chloritoid (WFe,Mg ¼ 1.5 kJ), two-site mixingwith regular solution for jadeite (Wjd,di ¼ 24 kJ,Wjd,ac ¼ 0 kJ), nonideal mixing for phengite andparagonite and regular solution model for garnet(Wpy,alm ¼ 2.5 kJ, Wgr,py ¼ 33 kJ).The jadeite + chloritoid + sodic amphibole sub-

assemblage without paragonite is found in severalsamples in the Orhaneli area (cf. Fig. 2). It allows an

estimation of minimum metamorphic pressures fromthe near-isobaric reaction (1). This equilibrium wascalculated for ferrous- and magnesian-endmembersusing analysed mineral compositions from five samples(cf. Table 1), and a paragonite activity of 0.9. Itindicates minimum pressures of c. 20 kbar for thetemperature range of 400–500 �C (Fig. 8). Similarminimum pressures of c. 22 kbar are also obtainedfrom the H2O-free reaction:

paragonite þ glaucophane þ muscovite¼ Jadeite þ magnesiochloritoid þ quartz

þ celadonite ð4Þ

The apparent absence of coesite places a maximumpressure of 27 kbar for the metamorphism in theOrhaneli region.The metabasite sample 104B contains gar-net + lawsonite + talc + quartz, which allows anestimation of the metamorphic temperature on thebasis of the reaction:

talc þ lawsonite ¼ garnet þ quartz þ H2O ð5ÞThis reaction indicates a metamorphic temperature of430 ± 20 �C at 24 kbar and at an activity of H2Oequal to one (Fig. 8). A lower activity of H2O wouldindicate an even lower metamorphic temperature.A similar metamorphic temperature of 430 �C was

Fig. 8. Pressure-temperature diagramshowing equilibria relevant to the estimationof the P–T conditions of the Orhaneligreyschists. Facies boundaries are afterEvans (1990); LBS, lawsonite blueschist;EPS, epidote blueschist; E, eclogite; AEA,albite-epidote amphibolite; GS, greenschistfacies. All reactions were calculated usingTHERMOCALCTHERMOCALC of Powell & Holland (1988) andHolland & Powell (1998). The garnet-form-ing equilibria represent an average of theferrous and magnesian endmember reactionsfor almandine and pyrope activities of 0.26and 0.00061, respectively, and the data fromthe sample 5080. As paragonite does notoccur with jadeite-chloritoid-sodic amphi-bole subassemblage, the dashed reactionslines in the Figure involving these mineralsprovide minimum pressure estimates.Abbreviations: ab, albite; clin, clinochlore;coe, coesite; ctd, chloritoid; cz, clinozoisite;di, diopside; fctd, Fe-chloritoid; fgl, ferro-glaucophane; fta, ferrotalc; ga, garnet; gl,glaucophane; jd, jadeite; ky, kyanite; law,lawsonite; mctd, Mg-chloritoid pa, parago-nite; q, quartz; ru, rutile; sa, sodic amphi-bole; ttn, titanite; ta, talc; v, H2O.

7 64 A . I . O K A Y

estimated in the blueschists from the Kocasu area,25 km farther west using a different garnet-formingreaction in a calc-silicate blueschist (Okay & Kelley,1994). The low metamorphic temperatures estimated inthe Orhaneli region are also supported by petrologicalcomparison with other blueschist and eclogite terranes,as discussed below.In most high-pressure metamorphic terranes chlor-

itoid in the metasediments is accompanied by garnet,and the associated metabasites contain sodicamphibole + garnet + epidote ± omphacite para-genesis (e.g. Alps: Chopin, 1981b; Spear & Franz,1986; Vuichard & Ballevre, 1988; Oman: Goffe et al.,1988; El-Shazly & Liou, 1991; New Caledonia: Ghentet al., 1987; Alaska: Brown & Forbes, 1986). In thehigh-pressure metapelites garnet forms in the tem-perature interval 450–500 �C through continuous re-actions involving chloritoid, sodic amphibole, jadeite:

chloritoid þ sodic amphibole¼ garnet þ jadeite þ quartz þ H2O ð6Þ

chloritoid þ sodic amphibole þ quartz¼ garnet þ paragonite þ H2O ð7Þ

chloritoid þ jadeite þ quartz¼ garnet þ paragonite þ H2O ð8Þ

chloritoid þ chlorite þ quartz ¼ garnet þ H2Oð9Þ

Schliestedt & Okrusch (1988), and Guillot et al. (1997)inferred reaction (8) in the sodic metapelites in theCyclades and in the Himalaya, respectively, on the basisof chloritoid and jadeite inclusions in garnet. Themetamorphic temperatures and pressures estimated forthe chloritoid-garnet bearing metasediments arec. 450 �C, 17 kbar in Peloponnese (Theye & Seidel,1991), c. 460 �C, 12 kbar in the Seward peninsula,Alaska (Forbes et al., 1984; Patrick & Evans, 1989),c. 470 �C, 15 kbar in the Cyclades (Schliestedt, 1986;Okrusch & Brocker, 1990), c. 475 �C, 8 kbar in Oman(Goffe et al., 1988). In the Orhaneli area, the greyschistsdo not contain garnet and the associated metabasitesare characterized by the sodic amphibole + lawsoniteparagenesis, suggesting that the metamorphic temper-atures were < 470 �C, which is compatible with thegeneral observation that garnet starts to form in peliticmetasediments at c. 450 �C (Yardley, 1989; p. 86). Atpressures >16 kbar, the transition from lawsonite–blueschist to epidote–blueschist and eclogite facies isestimated to occur at between 450 and 500 �C (Fig. 8,Evans, 1990). Thus, various lines of evidence suggestthat the metamorphic temperatures in the Orhaneliblueschists were below 450 �C, in accordance with thetemperature estimate from reaction (5).The temperature at which garnet would have formed

in the sodic metapelites in the Orhaneli region is esti-mated by using an appropriate garnet composition

that would have coexisted with chloritoid and sodicamphibole at higher temperatures. Sodic metapelitesfrom the Peloponnese contain chloritoid + sodicamphibole + garnet + paragonite assemblages withchloritoid compositions similar to that in the Orhaneliarea (Theye & Seidel, 1991). The estimated tempera-ture in the Peloponnese is only slightly higher than thatin the Orhaneli area (450 ± 30 �C) so that there willbe little change in the KD values [(Fe ⁄Mg)ctd ⁄(Fe ⁄Mgga)] between the two areas. Garnet-formingequilibria in the sample 5080 from the Orhaneli regionwere calculated using a garnet composition from thePeloponnese (sample P80 ⁄ 36) that coexists with chlo-ritoid of similar composition as that in the sample 5080(Fig. 8). The equilibria indicate that garnet would haveformed in the metapelites of the Orhaneli region ataround 450 �C through reaction (6).The arguments above suggest a 24 ± 3 kbar pres-

sure and 430 ± 30 �C temperature for the peakblueschist facies conditions in the Orhaneli area. Thetemperature and pressure estimates assume the pres-ence of a free hydrous phase (aH2O ¼ 1). The abun-dance of hydrous minerals (e.g. phengite, lawsonite,sodic amphibole, chloritoid), and textural and min-eralogical equilibrium at these relatively low tempera-tures argue for the presence of a fluid phase. A highH2O activity is also indicated by the reaction (4), whichdoes not involve H2O. The frequent presence of laws-onite in the greyschists indicates that the fluid consistedmainly of H2O (XCO2 < 0.03, Nitsch, 1972).The blueschists from the western Tavsanlı Zone are

characterised by prograde rather than retrograde tex-tures and minerals (Okay, 1984). This is also true forthe Orhaneli region, where the rimward increase in theglaucophane component in zoned sodic amphibolesuggests increasing pressures and temperaturesaccording to reaction (1). Similarly the zoning ingarnet in the metabasite is a typical prograde feature.A greenschist overprint, e.g. barroisite rims aroundsodic amphibole or breakdown of lawsonite to clino-zoisite, is absent in the western part of the TavsanlıZone. The estimated P–T conditions in the Orhaneliregion are close to the formation temperatures ofgarnet. Therefore, it is remarkable that although thegarnet-forming reaction (7) has a positive slope in theP–T field, garnet has not formed in the retrograde pathof the Orhaneli blueschists. This suggests that tem-perature in the blueschists in the Orhaneli region didnot increase during the exhumation. The late mineralsin the blueschists are chlorite, which has formed at theexpense of sodic amphibole and chloritoid, and minoralbite. Both the prograde and retrograde paths ofthe blueschists must have stayed in the lawsonite–blueschist facies field.

CONCLUSIONS

Blueschists metaclastic rocks and marble form athick coherent stratigraphic sequence in the Orhaneli

J A D E I T E – C H L O R I T O I D – L A W S O N I T E B L U E S C H I S T S 76 5

area in northwest Turkey. The jadeite + chlori-toid + glaucophane + lawsonite + quartz + pheng-ite assemblage in the metasediments provides a P–Testimate of 24 ± 3 kbar and 430 ± 30 �C, giving aT ⁄P ratio of 18 �C kbar)1. The overburden on theblueschists probably consisted of metasediments,accretionary complex and oceanic lithosphere (Okayet al., 1998). Taking an average density of 3.1 g cm)3

for the overburden, the 24-kbar pressure translatesinto 79 km depth. For a peak temperature of 430 �C,this would indicate a geotherm of 5.4 �C km)1, one ofthe lowest geotherm recorded in the high-pressurerocks. It is important to stress that this geotherm is notfor blocks in a melange but for a coherent blueschistterrane, several tens of square kilometres in size and atleast one kilometre thick. Peak P–T estimates fromselected blueschist and low-temperature eclogite terr-anes are shown in Fig. 9. Most HP ⁄LT metamorphicregions appear to be characterized by geotherms of 8to 14 �C km)1. However, this could partly reflect theproblems of estimating metamorphic pressures inblueschist terranes that lack metapelitic lithologies. Itis significant that the lowest geotherms occur inregions, such as the Peloponnese (Theye & Seidel,1991), where metapelitic mineral assemblages are usedin the estimation of metamorphic pressures. The recent

proliferation of coesite-bearing metamorphic terranesis probably another indication that very low geothermsin HP ⁄LT metamorphic terranes are more commonthan realized.The thermal structure of the subducting lithosphereis controlled by the shear and radioactive heating,geometry and rate of subduction and the thermalstructure of the downgoing slab. Atlantic ocean-floor anomaly data indicate a convergence rate ofc. 3 cm y)1 during the Cretaceous between Africa andEurasia in the vicinity of northwest Turkey (Patriatet al., 1982; Livermore & Smith, 1984). For this rate ofconvergence, and for a wide-range of other subductionzone parameters (cf. Peacock, 1992), the 5 �C km)1

geotherm suggests low shear stresses of c. 200 baralong the subduction interface.Unstable minerals such as jadeite are preserved inthe Orhaneli blueschists indicating that the rocks havestayed largely dry during their exhumation, presum-ably the fluid phase was expelled from the rocks atpeak temperatures. However, even when fluids locallyreached the blueschists during exhumation, onlychlorite and sericitic white mica formed at the expenseof glaucophane and chloritoid, rather than barroisiteor epidote. In this aspect the western part of theTavsanlı Zone is more typical of high-pressure belts

Fig. 9. Peak P–T conditions of blueschistand eclogite facies metamorphism for selec-ted areas. Note that the P–T uncertanties areusually several times larger than that indi-cated by the ellipses. The dotted lines are thefacies boundaries after Evans (1990). Thedata sources are: Franciscan Complex:Diablo Range (Ernst, 1993; Dalla Torreet al., 1996), Ward Creek (Brown, 1977;Maruyama et al., 1986), Yolla Bolly terrane(Jayko et al., 1986); Stuart Fork terrane,California (Goodge, 1989), South ForkMountain schist (Brown & Ghent, 1983);western Crete and Peloponnese, Greece(Theye & Seidel, 1991); Oman (Goffe et al.,1988; El-Shazly & Liou, 1991); SewardPeninsula, Alaska (Forbes et al., 1984; Pat-rick & Evans, 1989); Sifnos, Greece (Schlie-stedt, 1986); Alanya, Turkey (Okay, 1989);Margarita, Venezula (Maresch & Abraham,1981); New Caledonia (Clarke et al., 1997;Carson et al., 1999); Tauern Window, Alps(Holland, 1979; Spear & Franz, 1986);Engadine Window (Bousquet et al., 1998);Himalaya (Guillot et al., 1997); Tianshan(Klemd et al., 2002) Spitsbergen (Hirajimaet al., 1988); Tavsanlı Zone, Turkey: Kocasu(Okay & Kelley, 1994), Orhaneli (this study).For other symbols see Fig. 8.

7 66 A . I . O K A Y

produced during the steady-state subduction of theoceanic lithosphere, which commonly escape a hightemperature overprint (e.g. Ernst, 1988). A fast overallexhumation rate was also not required for the preser-vation of the Orhaneli blueschists. Jadeite formed atc. 79 km depth during the Cretaceous (80 Ma) as stillresiding at 10 km depth during the Eocene (48 Ma),30 million years later.

ACKNOWLEDGEMENTS

I thank C. Chopin, N. Harris and B. Arman for the useof the electron microprobe, and R. Klemd, B. Natalinand J. Schumacher for constructive reviews. Thefieldwork in the Orhaneli region was supported by theTUBITAK grant YDABCAG 419 ⁄G.

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Received 4 December 2001; revision accepted 15 May 2002.

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